A bidentate-anionic-group strategy for enhancing electron–phonon coupling and vibronic fluorescence in rare-earth crystals

Abstract

Electron–phonon coupling, representing the energy transfer between electron states and lattice vibrations, is a fundamental physical cornerstone for many interesting phenomena. Phonon-assisted electronic transition manifests as fluorescence emission, providing a good platform for studying electron–phonon coupling. Herein, we obtained K₃Gd(BO₃)₂ and Li₃K₉Gd₃(BO₃)₇ crystals and studied the relations of the coordination type between the central Gd³⁺ ions and the [BO₃]³⁻ ligand. Using Gd³⁺ ions as a fluorescence probe, we found that the bidentate-BO₃ group, better than monodentate-BO₃, can greatly enhance the electron–phonon coupling and phonon-assisted fluorescence at longer wavelengths. Compared with K₃Gd(BO₃)₂, the Huang–Rhys S factor of Li₃K₉Gd₃(BO₃)₇ realized about triple improvement (from 0.057 to 0.163), which is attributed to the increased bidentate–BO₃ ratio (from 50% in K₃Gd(BO₃)₂ to 100% in Li₃K₉Gd₃(BO₃)₇). More impressively, this bidentate-anionic-group strategy can be extended to rare-earth phosphates, in which KGd(PO₃)₄ (without bidentate-PO₄) and K₃Gd(PO₄)₂ (with 50% bidentate-PO₄) exhibit S factors of 0.08 and 0.133, respectively. These discoveries provide some potential fluorescence matrices and a helpful guideline for searching new rare-earth hosts with strong electron–phonon coupling for tunable broadband vibronic lasers.